Electrophoretic determination of lactate dehgydraogenase (LD) isoenzymes in serum samples of medical and surgical patients improved the diagnostic discrimination of myocardial infarction in these patients. Applications of the same LD isoenzyme test to the analysis of serum samples from patients with testicular tumors provided useful information for the detection of the occurence of these tumors and their metastasis. The same test was employed for the determination of LD isoezyme pattern charecteristic of various types of normal and of leukemic cells, to serve as markers for the early detection and follow-up of treatment of leukemia patients. Charactristic serum LD-isoezyme patterns have been established in lymphoma patients that aid in the diagnosis of this disease.